test: make assert_return() critical by default on fuzzer and unit tests

Several test cases intentionally trigger assert_return(). So, to avoid
the entire test fails, this introduces several macros that tentatively
make assert_return() not critical.

src/fuzz/fuzz.h

@@ -31,6 +31,7 @@ static inline bool outside_size_range(size_t size, size_t lower, size_t upper) {
 static inline void fuzz_setup_logging(void) {
         /* We don't want to fill the logs and slow down stuff when running
          * in a fuzzing mode, so disable most of the logging. */
+        log_set_assert_return_is_critical(true);
         log_set_max_level(LOG_CRIT);
         log_parse_environment();
         log_open();